Content

So uso pointed out a very interesting part of my proposal:
On-the-fly validation when writing/editing XML-files.

The XML-structure is meant to have arbitrary data in the forms. Well, not
really: if the field is of arch "xml", it's content hat to be valid xml. Even
more: the allowed content depends on the field type (fomr, menu, tre, ...)
If this could be checks as early as editing, this saves developer time :-)